The diffusion transfer photographic method utilizing silver halide, for example, has been known. In accordance with this method, a light-sensitive element containing an exposed silver halide photographic emulsion is superposed on an image-receiving element containing a silver-precipitating agent and then an alkaline processing solution containing a solvent for silver halide is introduced between the light-sensitive and image-receiving elements in the presence of a developing agent, whereupon a positive silver image pattern is formed directly on the image-receiving element.
In this method, the unexposed silver halide emulsion is dissolved in the silver halide solvent and, as a silver ion complex, introduced in the alkaline processing solution. Thereafter, it is transferred to the image-receiving element and precipitated therein in the silver image pattern by the action of the silver-precipitating agent, resulting in the formation of the direct positive image.
However, the silver image pattern thus formed in the image-receiving element has a disadvantage in that it is readily discolored or fades during the storage.
In order to overcome the above defect, an improved method has been proposed, as described in Japanese Patent Publication No. 5392/71, U.S. Pat. No. 3,533,789, and British Pat. No. 1,164,642, in which a water-soluble polymer solution containing an alkali-neutralizing component is coated on the surface of the silver image. This improved method, however, still has disadvantages in that it takes a long time until the aqueous polymer solution coating completely dries and, during this period, the coating surface is adhesive and sticky, such that fingerprints or dust often attach thereto. Moreover, it is troublesome to coat such a solution on the silver image pattern.
Japanese Patent Publication No. 44418/81 discloses an image-receiving element comprising (1) a cellulose ester, polyvinyl ester, or polyvinyl acetal layer containing a compound which is diffusible and capable of changing the properties of the silver image, said layer being hydrolyzable and upon hydrolysis, becoming permeable to alkalis, and (2) a regenerated cellulose layer containing a silver-precipitating agent. Furthermore, as the compound which is diffusible and capable of changing the properties of the silver image, organic mercapto compounds are described.
Japanese Patent Publication No. 21140/81 and Japanese Patent Application (OPI) No. 500431/81 (the term "OPI" as used herein means a "published unexamined Japanese patent application") disclose that the silver image is prevented from fading by application of noble metal compounds onto the silver image.
These compounds, however, fail to sufficiently stabilize the silver image obtained by the diffusion transfer method. That is, even if the compounds are added, discoloration and fading inevitably occur. It has therefore been desired to develop a novel element having increased stability of the silver image.